Method and device for determining and compensating for the tilting of the spectrum in an optical fiber of a data transmission path

ABSTRACT

A method is provided for determining and setting the tilting of the spectrum of light signals in an optical fiber of an optical data transmission path having at least one part for varying the tilting of the spectrum, wherein the light signals are amplified by at least one optical amplifier and a portion of the amplified light signals is extracted, the extracted light signals are then partially guided through an influencing element with a known frequency-dependent intensity influence, the influencing element being an amplifier, a waveguide structure or a fiber with an amplifying action, the total intensity of the extracted light signals is then measured upstream and downstream of the influencing element prior to the extracted light signals being guided through the influencing element, and the control criterion is determined, based on the known frequency-dependent intensity influence of the influencing element and the measured total intensity, for setting the tilting via which the part for varying the tilting is controlled.

BACKGROUND OF THE INVENTION

It is known that power is transferred from higher to lower frequencies(from lower to higher wavelengths) and thus between data transmissionchannels in optical fibers by stimulated Raman scattering (SRS). Thus,the original frequency spectrum of the light signal is “tilted”. Thisreduces the received power of the channels with short wavelengths; thus,increasing their bit error rate. It is also known to measure the tiltingof the spectrum of light signals that are guided through optical fibers,particularly of optical data transmission paths, and to counteract thistilting by appropriate filtering or amplification.

In order to determine this tilting, use is made in the prior art of acomplicated spectrally resolving measuring technique that cannot bewidely applied because of the expensive and bulky measurementtechnology.

Furthermore, there is known from U.S. Pat. No. 5,818,629 a method and anarrangement for determining a mean wavelength (“momental wavelength”) ofthe transmitted light signals, and a control, dependent thereon, of anoptical amplifier for compensating for the tilting of the spectrum ofthe transmitted light signals, in the case of which the “momentalwavelength” of the injected light signals is determined in a “monitoringdevice” (see FIG. 1). The mean wavelength (“momental wavelength”) isused to determine the tilting of the spectrum of the injected lightsignals within the optical transmission system, and to control theamplification of the optical amplifier so as to virtually compensate forthe determined tilting of the spectrum. Such a method requires thetechnically complicated determination of the mean wavelength and theevaluation thereof.

It is, therefore, an object of the present invention to develop a methodthat can determine the tilting of the spectrum in an optical fiber in asimple and quick way without a spectrally resolving measuring technique.

SUMMARY OF THE INVENTION

In consideration of the above, the inventors have made the followingfindings:

As a consequence of the stimulated Raman scattering (SRS), power istransferred from channels with shorter wavelengths to channels withlonger wavelengths. Some of the channels thus experience an additionalattenuation, while the others experience through this nonlinear effectan amplification counteracting the fiber attenuation. This amplificationor additional attenuation is a function of time. However, this aspectcan be neglected in the case of strongly differing group delays betweenthe interacting channels, which is frequently the case when use is madeof SSMF (Standard Single Mode Fiber). Nevertheless, the result isprecisely different mean powers for the individual channels as from thewavelength dependence of the gain in EDFA (Erbium Doped FiberAmplifiers). This effect is termed “tilting” of the spectrum. It ispossible, in principle, for the gain of an EDFA to be set specificallysuch that this effect is countered. However, compensating for anytilting in a data transmission path requires a simple method to be foundfor determining the tilting.

It is also possible, in principle, to determine a linear tilting, thatis to say a first-order tilting, via the information from two totalintensities in the spectrum respectively after the passage through atleast one filter with a known frequency-dependent transmissioncharacteristic, or at least one amplifier with a knownfrequency-dependent influencing characteristic, designated in general asinfluencing element below. Tiltings of higher order, that is to saynonlinear tiltings, can be approximated correspondingly by anappropriately high number of measurements of total intensities after thepassage through other known frequency-dependent influencing elements ineach case.

In order to determine the spectral tilting of a signal, it issufficient, in principle, to extract light at a site and to determinethe total intensity after the passage of two influencing elements; forexample, filters or frequency-dependent amplifiers. One of these filtersalso can be reduced to an all-pass filter without phase response, suchthat it can just as well be removed. One influencing element and twomeasuring sites then suffice here for the total intensity, in order todetermine the spectral tilting of the signal. However, if the totalintensity of a signal is already known from other information before thepassage through an influencing element, a single influencing element anda single measuring site of the total intensity downstream of thisinfluencing element also suffice.

Signal tiltings in a signal path and tilting caused by the EDFA arefundamentally similar if a flat input spectrum is presupposed. However,in a transmission system, the spectrum at the transmitter end isdeliberately predistorted such that the signal tilting downstream andupstream of the EDFA must be determined in order to determine thetilting by the EDFA, since otherwise the information about the inputsignal is not to hand.

In accordance with these inventive ideas outlined above, the inventorspropose to improve the known method for determining the tilting of thespectrum of light signals in an optical fiber of an optical datatransmission path by virtue of the fact that the optical datatransmission path has the tilting of the spectrum, and the light signalsare amplified by at least one optical amplifier, and a portion of theamplified light signals is extracted. The extracted light signals arepartially guided through an influencing element with a knownfrequency-dependent intensity influence. Furthermore, the totalintensity of the extracted light signals is measured upstream anddownstream of the influencing element, and the total intensity of thelight signals is measured before the amplification. Use is made in thiscase as influencing element (11) of an amplifier or a waveguidestructure or fiber with an amplifying action. There is determined on thebasis of the known influence of the influencing element (11) and themeasured total intensities a control criterion for setting the tiltingvia which the tilting capability is controlled.

In a particular embodiment of the method, it is provided that use ismade, as influencing element, of a settable optical filter and/or afrequency-dependent amplifier, it preferably being possible for this tobe an EDFA (Erbium-doped Fiber Amplifier). It is also possible to makeuse as amplifier of other waveguide structures doped with rare earths. AMach-Zehnder with adjustable time delay in one branch or settableintensity division onto the two branches is an example for a settableoptical filter.

In accordance with the teachings of the present invention, the measuringmethod represented above also can be used for a method for setting orcompensating for the tilting of the spectrum of light signals in anoptical fiber of an optical data transmission path. This tilting can bevaried or compensated for by virtue of the fact that one or moresettable filters or attenuators and/or the frequency dependence of theamplification of one or more optical amplifiers, such as EDFA or otherwaveguide structures doped with rare earths, are set in such a way thatthey counteract the tilting produced on the transmission path.

It can be provided here according to the present invention that the partfor varying the spectrum is a frequency dependent optical amplifier,preferably a waveguide structure doped with rare earths, a fiber or anEDFA, it being possible for the frequency dependence of theamplification of the waveguide structure or fiber to be set by varyingthe pump power in such a way that is opposes the original tilting.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a shows a two-stage amplifier with reduction of the SRS influenceby tilting of the gain.

FIG. 1 b shows a transmission characteristic of a filter used.

FIG. 2 a shows an increase of the mean gain as a function of thetilting.

FIG. 2 b shows errors as a function of the tilting.

FIG. 3 shows a data transmission path with controllable gain tilting inorder to compensate for the SRS.

FIG. 4 shows an example of linearly tilted spectra and a linearfrequency response of a filter.

FIG. 5 shows measured powers downstream of the filter from FIG. 4, inrelation to the rise in the tilting of the spectrum.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a shows how the SRS influence can be reduced by tilting the gainof an EDFA, the transmission characteristic of the filter used beingillustrated in FIG. 1 b therebelow.

Since optical data transmission paths can be of very different design,and the spectral power distribution can change during operation, only avariable, or settable, “gain tilting” makes sense. It is assumed in thefollowing considerations that the mean population inversion of an EDFAin the initial state is selected such that minimal differences in gain,without the use of a filter, occur that are further largely completelyeliminated with the aid of a filter.

In order to counteract the effect of the SRS, channels must experience agreater amplification for shorter wavelengths than for longerwavelengths. Precisely this effect occurs when the mean populationinversion of the EDFA is increased, which is illustrated in FIGS. 1 aand 1 b. The gain profile is completely flat in the initial state. Inorder to achieve this, a filter with the transmission characteristic 3shown in FIG. 1 b was adopted. If the pump power in one or bothamplifier stages 7 is now increased in the case of an EDFA design asshown in FIG. 3, the mean population inversion increases and the desiredgain tilting occurs. Such a possible gain tilting as a function of theset power is also illustrated in the spectra of FIG. 4.

As FIGS. 1 a and 1 b show, however, this compensation method causes twodifficulties. If the power of the equidistant channels is plottedlogarithmically against their wavelength, the straight line 1 is yieldedto a very good approximation when only the SRS is acting. The profile ofthe gain 2 of an EDFA does not, however, exhibit a linear profile as afunction of the wavelength, and so no complete compensation is possible.In the example shown for an EDFA with 30 dB gain in the initial state,the pump power was set so as to result in a difference in gain 4 of atmost 3 dB. Because of the nonideal shape of the curve, power differences5 of 0.9 dB occur after the action of the SRS between the channels. Thisdeviation from the ideal shape of the curve can, however, be compensatedfor by inserting appropriate filters into the path at a few points. Insome circumstances, even the setting of different transmit powersalready suffices to obtain equal power levels and/or signal-to-noisepower ratios at all the receivers. A further disadvantage of the methodis that the mean gain likewise increases. This can be compensated for byincreasing the attenuation of the inserted attenuator. The error turnsout to be substantially smaller when the channels are displaced tohigher wavelengths by approximately 10 nm. The starting point in theexample was a wavelength range of 1570 nm to 1605 nm, the so-called Lband. However, the method also can be applied for other wavelengthranges.

The increase in the mean gain is illustrated in FIG. 2 a, and the“error” occurring is illustrated in FIG. 2 b as a function of thetilting. It may be seen that the power differences occurring owing tothe SRS can be compensated for to approximately ⅔. In the initial state,the gain in decibels is

${G_{opt} = {\frac{10}{\ln\; 10}{L \cdot \left\{ {{\left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot {\overset{\_}{N}}_{opt}} - {\sigma^{a}(\lambda)}} \right\}}}},$L standing for the total length of the doped fiber, σ^(e)(λ) andσ^(a)(λ) representing the coefficients, dependent on the wavelength λ,for emission and absorption, respectively, and N _(opt) representing themean population inversion in the initial state.

It is assumed below that the differences in gain are completelycompensated for in the initial state with the aid of a filter. If themean population inversion is now increased by the value Δ N, the resultfor the gain is

$G_{comp} = {\frac{10}{\ln\; 10}{L \cdot {\left\{ {{\left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot {\overset{\_}{N}}_{opt}} + {{\left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot \Delta}\overset{\_}{N}} - {\sigma^{\sigma}(\lambda)}} \right\}.}}}$By comparison with the initial state, an increase in gain by

${\Delta\; G_{comp}} = {\frac{10}{\ln\; 10}{L \cdot \left\lbrack {{\sigma^{e}(\lambda)} + {\sigma^{a}(\lambda)}} \right\rbrack \cdot \Delta}\overset{\_}{N}}$is thus to be recorded. The gain tilting effected by an increase in themean population inversion can, therefore, be described by a functionƒ(λ) that is fixed by the active cross section and is still to bemultiplied by a factor. The last equation above makes it plain that thecompensation of the SRS cannot be improved when the starting point isanother initial state of the EDFA.

In order to be able to use this method in a commercial transmissionsystem, it is necessary to have available a suitable rule that can beimplemented easily.

As already set forth, a unique relationship exists between the increasein the internal gain and the tilting. The internal gain can bedetermined from the gain measured between input and output, by furtheradding the attenuation of an inserted attenuator. A controlled variablefor the tilting is indirectly obtained thereby.

A grave difficulty results, nevertheless. Since no measuring device isgenerally available for spectrally resolved measurement, only the totalpower is known at the input and output of the amplifier, but not howthis is distributed over the individual channels. It is thus impossibleto determine a mean gain as unique reference variable. A simple solutionto this problem comes from the extended EDFA design shown in FIG. 3.Here, it is not only the total power that is measured at the input andat the output of the amplifier, but also the power at a specificwavelength. It is, therefore, possible to determine uniquely the gainfor this wavelength channel, and thus also the tilting.

In order to reduce the influence of nonlinear fiber effects, variousmethods can be applied for occupying the available wavelength rangedepending on the type of fiber used. The above-described design of thepresent invention leads to restrictions, since the wavelength channelused to measure the gain must always be in operation.

A possible solution that circumvents this restriction can be set forth,as illustrated in FIG. 3.

FIG. 3 shows a two-stage amplifier including a data transmission pathwith controllable gain tilting intended for compensating for the tiltingcaused by stimulated Raman scattering (SRS), with two controllableamplifier stages (EDFA) 7. For the purpose of clarity, the associatedelectronic arrangement is not illustrated. Upstream of the firstamplifier stage 7, a component signal is extracted via the first coupler6, and a first photodiode 10 is used to measure the unfiltered totalintensity and, after filtering by the known frequency-dependent filter11, to measure the filtered total intensity. In accordance with thefollowing description, these data are used to determine the inputtilting of the signal into the first amplifier stage 7. Locateddownstream of the first amplifier stage 7 is a further coupler 6 and aphotodiode 10 for controlling the gain of the first amplifier stage 7 incooperation with the total intensity measured at the input.Subsequently, the signal passes a settable frequency-independentattenuator 8, a further coupler 6 which, in turn, extracts a componentsignal at the input of the second amplifier stage 7 and feeds itunfiltered to a photodiode 10 for measurement. Downstream of the secondand last amplifier stage 7, once again, the resulting signal ispartially extracted and fed for measurement to a measuring arrangement 9with a photodiode 10 without prefilter and a photodiode 10 with anupstream filter 11. The tilting of the signal exiting the amplifierarrangement is determined in the way according to the present inventionby the last measuring arrangement, and the tilting is correspondinglykept within the desired bounds or completely compensated for by varyingthe inversion of the EDFA, that is to say by controlling the gain of theamplifier stages 7. The settable attenuator 8 serves a purpose in thiscase of reducing the gain of the total amplifier, if appropriate in afrequency-independent fashion, or of raising it by retracting a presetattenuation.

Thus, in this design it is not the task of the illustrated filter 11 toselect an individual channel, but to simulate in its attenuationresponse the wavelength dependence of the gain tilting, that is to saythe function f(λ) except for a constant of proportionality. Itstransmission characteristic isT(λ)=exp{−α·f(λ)},in which the constant α may be known. If the powers of the N channelsare designated by P_(i) and their wavelengths by λ_(i), the powersmeasured at the input are

$P_{in} = {\sum\limits_{i = 1}^{N}\; P_{i}}$and, after filtering,

$P_{in}^{filt} = {\sum\limits_{i = 1}^{N}\;{{P_{i} \cdot \exp}{\left\{ {{- \alpha} \cdot {f\left( \lambda_{i} \right)}} \right\}.}}}$It holds correspondingly for the powers measured at the output of theamplifier that

$P_{out} = {G_{opt} \cdot {\sum\limits_{i = 1}^{N}\;{P_{i}\mspace{14mu}\exp\left\{ {\chi \cdot {f\left( \lambda_{i} \right)}} \right\}}}}$and, after filtering,

$P_{out}^{filt} = {G_{opt} \cdot {\sum\limits_{i = 1}^{N}\;{P_{i}\mspace{14mu}\exp{\left\{ {\left( {\chi - \alpha} \right) \cdot {f\left( \lambda_{i} \right)}} \right\}.}}}}$

The constant χ determines the degree of gain tilting and is to bedetermined below. The first step for this purpose is to expand theexponential function in a series and truncate it after the second-orderterm. This results in the system of equations

${\frac{P_{out}}{G_{opt}} - P_{in}} = {{\chi \cdot {\sum\limits_{i = 1}^{N}\;{{f\left( \lambda_{i} \right)} \cdot P_{i}}}} + {\frac{\chi^{2}}{2} \cdot {\sum\limits_{i = 1}^{N}\;{{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}$${\frac{P_{out}^{filt}}{G_{opt}} - P_{in}} = {{\left( {\chi - \alpha} \right) \cdot {\sum\limits_{i = 1}^{N}\;{{f\left( \lambda_{i} \right)} \cdot P_{i}}}} + {\frac{\left( {\chi - \alpha} \right)^{2}}{2} \cdot {\sum\limits_{i = 1}^{N}\;{{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}$${P_{in}^{filt} - P_{in}} = {{{- \alpha} \cdot {\sum\limits_{i = 1}^{N}\;{{f\left( \lambda_{i} \right)} \cdot P_{i}}}} + {\frac{\alpha^{2}}{2} \cdot {\sum\limits_{i = 1}^{N}\;{{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}$consisting of three equations in which the three unknowns

$\chi,{\sum\limits_{i = 1}^{N}\;{{{f\left( {\lambda\; i} \right)} \cdot P_{i}}\mspace{31mu}{and}\mspace{31mu}{\sum\limits_{i = 1}^{N}\;{{f^{2}\left( \lambda_{i} \right)} \cdot P_{i}^{2}}}}}$are obtained. The gain G_(opt) in the initial state is known from thedesign and dimensioning of the EDFA. It is, therefore, possible todetermine the target variable χ uniquely:

$\chi = {\alpha \cdot {\frac{P_{out} + P_{out}^{filt} - {\left( {P_{in} + P_{in}^{filt}} \right) \cdot G_{opt}}}{P_{out} - P_{out}^{filt} + {\left( {P_{in} - P_{in}^{filt}} \right) \cdot G_{opt}}}.}}$

The series expansion was terminated after the second-order term in orderto keep the outlay low. As such, the exponential function can beapproximated only within a bounded value range. If this value range isto be enlarged, terms of higher order likewise can be taken intoaccount, there being a need to use further photodiodes with differentupstream filters.

The Taylor series expansion yields a very good approximation of theexponential function for very small arguments, while greater deviationsoccur in the case of greater values. Consequently, the suggestion is toadapt the factor ½ in front of the second term such that the maximumerror occurring becomes minimal within the desired value range. If thefactor ½ is replaced, for example, by 0.81, gain tiltings of up to 4.5dB can be set in conjunction with a maximum error of 0.18 dB.

In accordance with a further reaching aspect of the present invention,which leads to a particularly simple and elegant device for determiningthe tilting of the frequency spectrum, the following may be representedstill employing special application of the above-described principle:

When considering the tilting in the case of a measured spectrum S(λ) inthe wavelength region of λ_(start) to λ_(stop), it is possible todetermine it by a numerical analysis, and to characterize acharacteristic quantity for the tilting; for example, the first momentM₁ of the spectrum relative to the middle wavelength λ_(c) of thespectrum (λ_(c)=(λ_(start)+λ_(stop))/2):

M₁ = ∫_(−L/2)^(+L/2)xS(x + λ_(c)) 𝕕x   with   L = (λ_(Stop) − λ_(Start))

It is also possible to use other odd functions ƒ(x) (here, odd refers toƒ(x)=−ƒ(−x)):

V = ∫_(−L/2)^(+L/2)f(x)S(x − λ_(c)) 𝕕x

According to the present invention, instead of a complicated spectrallyresolved measurement of the spectrum S(λ) and a subsequent numericaldetermination, by spectral analysis, of the tilting, the spectrum isweighted with the frequency response G(λ) with the aid of an opticalfilter, and the aggregate output power P_(v) of the filter is measuredwith the aid of a simple photodiode. The weighting can be adapted inthis case to the expected tilting:

P_(V) = ∫₀^(+∞)G(λ)S(λ) 𝕕λ

Since the frequency response G(λ) and the spectrum S(λ) are greater than0, P_(v) is also greater than 0 even in the case of an untiltedspectrum. This offset is to be born in mind during use. Furthermore, thefrequency response G(λ) from: λ_(start) to λ_(stop) should be odd inrelation to G(λ_(c)) (here, odd refers toG(λ_(c)+x)−G(λ_(c))=−[G(λ_(c)−x)−G(λ_(c))]). Moreover, the monotonicedge of the filter frequency response should extend from λ_(start) toλ_(stop). Again, frequency responses of photodiodes or couplers can betaken into account in G(λ), if they would otherwise lead tofalsifications of the measurement result. A bandpass restriction to awavelength region to be considered (for example, from λ_(start) toλ_(stop)) likewise can be included in G(λ).

A number of linearly tilted spectra S(λ)=a·λ+b, illustrated as dashedlines, are shown as a functional example in FIG. 4. The total power

∫₀^(+∞)S(λ) 𝕕λof all the spectra is the same here, and so the same power would bemeasured via a photodiode independently of the tilting. If a filter withfrequency response G(λ) is now inserted, the power measured at thephotodiode becomes dependent on the tilting, as illustrated in FIG. 5,and it is thereby possible to use it as a measure of the tilting forcontrol tasks.

For example, a Mach-Zehnder interferometer with a cos²-type frequencyresponse can be used as suitable filter. In this case, the measuredvalue has an offset dependent on the aggregate power of the opticalsignal. Offset refers to the measuring device supplying a signal evengiven a vanishing tilting of the spectrum. This disadvantage can beavoided via an optical filter with two opposing frequency responsesG_(AB)(λ) and G_(AC)(λ) and if it holds that G_(AB)(λ)+G_(AC)(λ)=const.An example of implementation with two opposing frequency responses isthe use of the two outputs of a Mach-Zehnder interferometer. The tiltingand the total power of the signal can be determined simultaneously withthe aid of this design:

The tilting V is yielded from the difference between the measured valuesby:

V = P_(VC) − P_(VA) = ∫₀^(+∞)[G_(AC)(λ) − G_(AB)(λ)]S(λ) 𝕕λ

The offset of V vanishes in the case of G_(AB)(λ_(c))=G_(AC)(λ_(c)) fora linear frequency response.

The aggregate power P is calculated from the sum of the measured values:

P = P_(VC) + P_(V A) = ∫₀^(+∞)[G_(A C)(λ) + G_(A B)(λ)]S(λ)𝕕λ = const.∫₀^(+∞)S(λ) 𝕕λ

This variable is advantageously used as early as when controlling thepump laser diodes in fiber amplifiers, and now also can be used fornormalizing the tilting if the magnitude of the tilting, and not onlythe sign, is required.

The very simple and therefore cost-effective design of this measurementproves to be particularly advantageous in this solution illustrated,there being no need for spectrally resolved measurement. A decentralcontrol becomes possible, as a result of which the outlay on controlsoftware is reduced and the control rate is increased. Furthermore, theweighting can be adapted to a fundamentally known tilting function ofthe fiber amplifier, and to possible disturbances in the spectrum suchas, for example, tilting owing to SRS attenuation.

Thus, as a whole, the present invention exhibits a simple method fordetermining the tilting of the spectrum of an optical signal bymeasuring at least one total intensity subsequent to a passage of thesignal through an influencing characteristic, including the possibilityof using it to set the spectral tilting.

It goes without saying that the abovenamed features of the presentinvention can be used not only in the combination respectivelyspecified, but also in other combinations or on their own, withoutdeparting from the scope of the invention.

Indeed, although the present invention has been described with referenceto specific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

1. A method for determining and setting a tilting of a spectrum of lightsignals in an optical fiber of an optical data transmission path havingat least one part for varying the tilting of the spectrum, the methodcomprising the steps of: measuring a total intensity of the lightsignals before amplification; amplifying the light signals by at leastone optical amplifier; extracting a portion of the amplified lightsignals; partially guiding the extracted light signals through aninfluencing element with a known frequency-dependent intensityinfluence, the influencing element being one of an amplifier, awaveguide structure having a positive amplifying action, and a fiberhaving a positive amplifying action; measuring a total intensity of theextracted light signals directly before and directly after theinfluencing element, wherein a point of measurement that occurs directlybefore the influencing element is not further amplified prior to thepoint where the extracted light signal reaches the influencing element;and determining a control criterion, based on the knownfrequency-dependent intensity influence of the influencing element andthe measured total intensities, for setting the tilting via which thepart for varying the tilting is controlled.
 2. A method for determiningand setting a tilting of a spectrum of light signals in an optical fiberof an optical data transmission path as claimed in claim 1, wherein theinfluencing element further may be one of a Mach-Zehnder interferometer,a dielectric filter, a fiber grating, and a wavelength-selective fusioncoupler.
 3. A method for determining and setting a tilting of a spectrumof light signals in an optical fiber of an optical data transmissionpath as claimed in claim 1, wherein the at least one optical amplifieris an Erbium Doped Fiber Amplifier.
 4. A method for determining andsetting a tilting of a spectrum of light signals in an optical fiber ofan optical data transmission path as claimed in claim 1, wherein use ismade of the at least one optical amplifier as the at least one part forvarying the tilting.
 5. A method for determining and setting a tiltingof a spectrum of light signals in an optical fiber of an optical datatransmission path as claimed in claim 4, wherein an existing tilting isone of compensated for completely, partially compensated for, and set toa tilting in an opposite direction.
 6. A method for determining andsetting a tilting of a spectrum of light signals in an optical fiber ofan optical data transmission path as claimed in claim 4, wherein atilting of a predefined magnitude is produced.
 7. A method fordetermining and setting a tilting of a spectrum of light signals in anoptical fiber of an optical data transmission path as claimed in claim1, wherein the at least one part for varying the tilting is a settablefrequency-dependent optical filter.
 8. A method for determining andsetting a tilting of a spectrum of light signals in an optical fiber ofan optical data transmission path as claimed in claim 7, wherein thesettable frequency-dependent optical filter is a Mach-Zehnderinterferometer with one of adjustable power division into its twobranches and settable time delay in at least one of its two branches.